Echo canceller

ABSTRACT

There is provided an echo canceller which can appropriately cancel an echo even when a low-frequency component is included in the signal to be passed. The echo canceller includes an echo replica forming means for forming an echo replica signal from a far-end input signal by using an adaptive filter including a filter section and a coefficient update section, and an echo cancellation means for removing an echo component in a near-end input signal by subtracting the echo replica signal from the near-end input signal. The echo canceller further includes an offset removal means for removing an offset component produced under an effect of low frequencies from the filter coefficient of the adaptive filter.

TECHNICAL FIELD

The present invention relates to an echo canceller, and, for example, issuitable for applying to an echo canceller which removes a line echooccurring in a hybrid circuit used in broadband voice telephony.

BACKGROUND ART

It is known that the echo canceller forms an echo replica signal byusing an adaptive filter and removes an echo component from a near-endinput signal by the echo replica signal. The echo canceller also causesthe filter coefficient of an adaptive filter to be updated and toconverge on a predetermined algorithm (e.g., LMS) to form an appropriateecho replica signal. A higher convergence speed of the filtercoefficient is preferable because the time for forming an appropriateecho replica signal is reduced.

In general, a control is made so that the filter coefficient is updatedin accordance with a far-end input signal and a near-end input signalafter echo component cancellation. It is conventionally known that noisesuch as a bias (DC offset) in a far-end input signal makes it impossibleto update the filter coefficient, thereby degrading the echocancellation capability.

A non-patent document 1 discloses technologies related to active noisecontrol instead of the echo canceller. It seems that the technologiesare related to bias compensation of the adaptive filter and can beapplied to the echo canceller.

Non-patent document 1: Nomoto, et al., “A Study on the Bias Compensationand the Variable Step Size Algorithms for the Filtered-X”, TechnicalReport of IEICE, DSP97-14 (1997-05)

DISCLOSURE OF INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

However, the technology described in non-patent document 1 has thefollowing problems.

1. Since the calculation for removing the bias for a desired signal isperformed for each sample, a large amount of calculation processing isrequired.

2. It is assumed that a bias component (the technology described innon-patent document 1 takes not only a direct-current component but alsoa gently varying component as the above-mentioned bias component) in adesired signal is always caused by noise, and the bias component isdirectly removed from the desired signal. Therefore, if this technologyis used for communication, any necessary low-frequency component, whichvaries gently, in a desired signal is removed as a bias component,thereby adversely affecting the sound quality.

In view of the foregoing, it is an object of the present invention toprovide an echo canceller which involves small amounts of computations,reduces the scale of hardware and the scale of software, and provides ahigh sound quality without degrading the components of the signal to bepassed even if a low-frequency component is included in the signal to bepassed.

MEANS FOR SOLVING THE PROBLEM

In order to solve the above mentioned problems, an echo cancelleraccording to the present invention includes: an echo replica formingmeans for forming an echo replica signal from a far-end input signal byusing an adaptive filter including a filter section and a coefficientupdate section; an echo cancellation means for removing an echocomponent in a near-end input signal by subtracting the echo replicasignal from the near-end input signal; and an offset removal means forremoving an offset component produced under an effect of low frequenciesfrom the filter coefficient of the adaptive filter.

EFFECTS OF THE INVENTION

An echo canceller which can be implemented according to the presentinvention involves small amounts of computations, can reduce the scaleof hardware and the scale of software, and can appropriately remove echoeven if a signal to be passed contains a low-frequency component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of an echo cancellerof the first embodiment;

FIGS. 2A and 2B are diagrams (No. 1) for describing the reason why anoffset removal section for removing an offset of a tap coefficient isprovided in the first embodiment;

FIGS. 3A and 3B1 to 3B3 are diagrams (No. 2) for describing the reasonwhy the offset removal section for removing the offset of the tapcoefficient is provided in the first embodiment;

FIGS. 4A, 4B1 to 4B3, and 4C1 to 4C3 are diagrams (No. 3) for describingthe reason why the offset removal section for removing the offset of thetap coefficient is provided in the first embodiment;

FIG. 5 is a block diagram showing a configuration of an echo cancellerof the second embodiment;

FIG. 6 is a block diagram showing a configuration of an echo cancellerof the third embodiment; and

FIG. 7 is a block diagram showing a configuration of an echo cancellerof the fourth embodiment.

EXPLANATION OF THE REFERENCE MARKS

8 adder; 10 double talk detector; 11 coefficient update section; 12filter section; 13, 22 offset removal section; 14, 14A, 14B, 14C echocanceller; 15, 45 adaptive filter; 16 counter; 20 sending-side FFTsection; 21 receiving-side FFT section; 30, 40 sending-side low-passfilter section; 31, 41 receiving-side low-pass filter section.

BEST MODE FOR CARRYING OUT THE INVENTION

(A) First Embodiment An echo canceller applied for removing a line echo

according to the first embodiment of the present invention will next bedescribed in detail with reference to drawings.

(A-1) Configuration of First Embodiment

FIG. 1 is a block diagram showing a configuration of the echo cancellerof the first embodiment. Referring to FIG. 1, an echo canceller 14 ofthe first embodiment includes an input terminal 1 of a digital audiosignal R_(in) from a talker at a remote end (hereafter referred to as afar end, not shown), an output terminal 2 for outputting an audio signalR_(out) from the far end to a receiver (hereafter referred to as a nearend), an input terminal 7 of a signal S_(in) from the near end, and anoutput terminal 9 of a signal S_(out) to the far end.

The signal R_(out) from the output terminal 2 of the echo canceller 14is converted to an analog signal by a digital-to-analog converter 3, andthen the signal is given through a hybrid circuit 4 to a telephone set5. An audio signal from the telephone set 5 passes the hybrid circuit 4and an analog-to-digital converter 6, and enters the input terminal 7 ofthe echo canceller 14. A part of the signal R_(out) from the outputterminal 2 passes the hybrid circuit 4 as it is and directly goes to theinput terminal 7, and the echo canceller 14 removes the echo component.

The echo canceller 14 includes an adder 8, a double talk detector 10, acoefficient update section 11, a filter section 12, an offset removalsection 13, and a counter 16. The coefficient update section 11 and thefilter section 12 form an adaptive filter 15.

In the first embodiment, the offset removal section 13 and the counter16 are added to the general configuration, and accordingly, the filtersection 12 is different in some degree.

The functions of the filter section 12, the offset removal section 13,and the counter 16 will be described in the following description of theoperation.

(A-2) Operation of First Embodiment

The operation of the echo canceller 14 of the first embodiment will nextbe described. In the subsequent explanation, the telephone set 5 isassumed to be an IP telephone set, which has sprung into wide use inrecent years, for example. Unlike the conventional telephone set, the IPtelephone set is not assigned a limited frequency band of the audiosignal.

For example, the audio signal of the conventional fixed telephone isassigned a limited frequency band of 300 to 3400 Hz (hereafter, thissignal is referred to as the conventional band signal). However, sincethe audio signal of the IP telephone set is not assigned a limitedfrequency band of 300 to 3400 Hz, a signal of a wider frequency band canbe received and sent. The international standard ITU-T G.722 specifies aspeech coding technique for communication in a band of 50 to 7000 Hz.This technique widens the frequency coverage and allows an audio signalhaving a high sound quality to be transferred. For example, the firstembodiment shows an example handling a signal of 20 to 7000 Hz(hereafter referred to as a broad-band signal). Of course, the frequencyband is not limited to this range.

A broad-band signal R_(in) input to the far-end input terminal 1 shownin FIG. 1 is supplied to the double talk detector 10, the filter section12, and the far-end output terminal 2. The functions of the double talkdetector 10 and the filter section 12 will be described later. A signalR_(out) output from the far-end output terminal 2 is converted to ananalog signal by the digital-to-analog converter 3, and the signal isoutput to the telephone set 5. In the meantime, a part of the signalR_(out) output from the far-end output terminal 2 is reflected by thehybrid circuit 4, converted to a digital signal Sin by theanalog-to-digital converter 6, and input to the near-end input terminal7. The signal S_(in) input to the near-end input terminal 7 is convertedto a signal S_(out) after echo cancellation, and sent through thenear-end output terminal 9 to the talker at the far end. When the outputsignal R_(out) of the far-end output terminal 2 is input to the near-endinput terminal 7, the talker, which is not shown, at the far end hearshis or her own voice as an echo component y, interfering with theconversation. The echo canceller 14 is provided to remove the echocomponent y.

In the echo canceller 14, the signal S_(in) containing the echocomponent y is input from the near-end input terminal 7 and enters theadder 8. The adder 8 subtracts an echo replica signal (pseudo-echosignal) y′, which has been created by the adaptive filter 15 asdescribed later, from the signal S_(in). The signal S_(out) (e), fromwhich the echo has been removed by the adder 8, is output to the outputterminal 9 to the talker at the far end. The signal S_(out) is routedthrough a path such as an IP network and output toward the telephone setof the talker at the far end, which is not shown. Thus, the signal, fromwhich the echo has been removed, reaches the talker at the far end.

The method of creating the echo replica signal y' will next bedescribed. An algorithm for generating an echo replica signal in thefirst embodiment is a known normalized least mean square algorithm. Ofcourse, another appropriate echo replica signal creation algorithm canbe used.

As described earlier, the coefficient update section 11 and the filtersection 12 form the adaptive filter 15. The operation of the adaptivefilter 15 will be described first. The signal R_(in) input from thefar-end input terminal 1 enters the filter section 12. The filtersection 12 is a known FIR (finite impulse response) filter. The tapcoefficient of the adaptive filter 15 is updated over time as describedlater.

Suppose that the m-th tap coefficient of the filter section 12 at time kis h(k,m). The value of the signal R_(in) from the far-end inputterminal 1 at time k is expressed as x(k). The filter section 12 createsthe echo replica signal y' as given by expression (1). A symbol Mrepresents the number of taps of the filter section 12, which can be256, but is not limited to 256. As indicated by expression (1), the echoreplica signal y′ is formed by a product-sum operation of the data ofthe past M samples of x(k) and the tap coefficient. $\begin{matrix}{y^{\prime} = {\sum\limits_{m = 0}^{M - 1}\quad{{h\left( {k,m} \right)} \cdot {x\left( {k - m} \right)}}}} & (1)\end{matrix}$

The tap coefficient of the filter section 12 is updated as given byexpression (2) in preparation for next filtering. The initial values ofh and x in expression (2) are 0. Further, μ denotes a constant fordetermining the tracking speed of the echo canceller 14 (0≦μ≦1). Thevalue of μ can be 0.5, for example, but the value is not limited to 0.5.When the tracking speed determination constant μ increases, the trackingspeed of the echo canceller 14 increases, but the echo cancellationcapability is degraded in a steady state. On the other hand, when thetracking speed determination constant μ decreases, the echo cancellationcapability of the echo canceller 14 is improved in a steady state, butthe tracking speed decreases. $\begin{matrix}{{h\left( {{k + 1},m} \right)} = {{h\left( {k,m} \right)} + {\mu\frac{{e(k)} \cdot {y(k)}}{\sum\limits_{i = 0}^{M - 1}\quad{x^{2}\left( {k - i} \right)}}}}} & (2)\end{matrix}$

e(k) in expression (2) denotes the output of the adder 8 (a residual ofthe echo subtraction). With y (k) and y′ (k) denoting the echo componenty and the echo replica signal y′ at time k respectively, expression (3)is given as follows:e(k)=y(k)−y′ (k)  (3)

As mentioned above, tap coefficient control in accordance withexpressions (2) and (3) uses the known “normalized least mean squarealgorithm.” It is a known fact that the tap coefficient h(k,m) isupdated in such a manner that the residual of echo subtraction e(k) (orits power) in expression (3) gradually approaches 0. The tap coefficientis updated (the filter converges) in such a manner that the echocomponent y is gradually reduced by the adder 8. The characteristics ofthe hybrid circuit 4, which is an echo path, are estimated by the tapcoefficient of the filter section 12, and the echo component y isremoved. If a near-end talker signal s is also input to the near-endinput terminal 7, the right-hand side of expression (3) becomes as shownin expression (4), which includes the near-end talker signal s, and thetap coefficient cannot be updated correctly. In expression (4), s(k)denotes the value of the near-end talker signal at time k.e(k)=y(k)−y′ (k) +s(k)  (4)

Therefore, if there is a near-end talker signal as in expression (4), acoefficient update must be stopped. The double talk detector 10 detectsa status as indicated by expression (4) or the like.

The double talk detector 10 receives the output signal x(k) from thefar-end input terminal 1 and the output e(k) from the adder 8. Thesmoothed value of the power of the signals is obtained in accordancewith expressions (5) and (6).pow_(—) x(k)=pow_(—) x(k−1)·δ+x ²·(1−67)  (5)pow_(—) e(k)=pow_(—) e(k−1)·δ+e ²·(1−67)  (6)

A constant 6 represents the degree of smoothness (1 >δ>0). If thesmoothness constant δis large, general variations in the signals x and eare reflected, and the effect of noise decreases. If the smoothnessconstant δis small, steep changes in the signals x and e are correctlyreflected, and the effect of noise increases. For example, δcan be 0.5,but δis not limited to 0.5.

The double talk detector 10 detects the presence or absence of thesignals (talkspurt or silence) from the smoothed values of powerpow₁₃x(k) and pow₁₃e(k) by a method as described later. When a signal isfound only on the side of the signal path from the far-end inputterminal 1 to the far-end output terminal 2, the coefficient of theadaptive filter 15 is updated. Otherwise, the double talk detector 10outputs a signal nt for stopping a coefficient update to the coefficientupdate section 11.

The double talk detector 10 determines whether a signal exists or not,as follows. When condition 1 is satisfied, the double talk detector 10outputs nothing to the coefficient update section 11 and allows acoefficient update. Condition 1:

pow₁₃x(k)>(silence threshold value), and

pow₁₃x(k)>pow₁₃e(k)+(margin value)

(silence threshold) can be−38 dBm, and (margin value) can be 6 dB, forexample, but (silence threshold) and (margin value) are not limited tothese values.

When condition 1 is not satisfied, it is determined that

-   1. the near-end input signal 7 has an input signal;-   2. the far-end input terminal 1 has no input signal;-   3. both the far-end input terminal 1 and the near-end input signal 7    have no input signal; or-   4. both the far-end input terminal 1 and the near-end    input signal 7 have an input signal.    Then, a coefficient update stop signal nt is output to the    coefficient update section 11. When the coefficient update stop    signal nt is given, the coefficient update section 11 does not    perform a coefficient update indicated by expression (2).

As has been described above, the double talk detector 10 does not updatethe coefficient in a state indicated by expression (4) (double-talkstate). Further, if the far-end input terminal 1 has no signal, no echooccurs, and the coefficient is not updated. Accordingly, the coefficientis updated in the state indicated by expression (3). The coefficient ofthe filter section 12 can be correctly converged by a known normalizedleast mean square algorithm. In the description given above, the doubletalk detector 10 detects talkspurt or silence of the input signal to thefar-end input terminal 1 and talkspurt or silence of the input signal tothe near-end input terminal 7, by using the smoothed values of power ofx(k) and e(k). Another appropriate method that can detect at leastsilence of the input signal to the far-end input terminal 1 andtalkspurt of the input signal to the near-end input terminal 7 (near-endtalker) can be used.

The function and operation of the offset removal section 13 representingthe characteristics of the echo canceller 14 of the first embodimentwill next be described.

It is conventionally known that a direct-current offset added to theinput signal degrades the echo cancellation capability of the echocanceller. This offset component has been believed to be caused by justthe characteristics of the analog-to-digital converter 6 or thebackground noise of the near-end input terminal 7.

When communication is constantly performed in a wide frequency band,such as by the broad-band telephone set, it has been found that the echocanceller is affected by the offset component or a component assumed tobe an offset even without the effect of the background noise oranalog-to-digital converter. This phenomenon will be described withreference to FIGS. 2A and 2B.

In the conventional general telephone communication, the frequency bandof the audio signal is limited to 300 to 3400 Hz. The lower limit of thefrequency band is about 300 Hz. Further, the conventional telephone setgenerally uses a sampling frequency of 8000 Hz. In the conventional art,the frequency that can be reproduced by the FIR filter described aboveis limited. The upper limit is a half of the sampling frequency, i.e.,4000 Hz, on the basis of a known sampling theorem. The lower limitdepends on the tap length of the FIR filter. With a sampling frequencyof 8000 Hz and an echo canceller with a tap length of 256, the FIRfilter can express the waveform of up to one cycle. That is, the lowestfrequency that can be expressed by the filter is 1/ (256x(1/8000))=31.25Hz.

Since the lowest frequency of the input signal of the conventionaltelephone set is 300 Hz, the echo canceller can represent the signalsufficiently. Accordingly, the tap coefficient of the filter of the echocanceller is not affected by a low-frequency offset. This state is shownin FIG. 2A.

Broad-band communication will next be described. Suppose that thesampling frequency is 16 kHz. If the echo canceller has a tap length of256 like the one described earlier, the lowest frequency that can beexpressed by the echo canceller is 1/(256×(1/16000))=62.5 Hz.

The filter section 12 of the echo canceller cannot express a frequencycomponent below the lowest frequency because the length of one cycleexceeds the filter tap length. The broad-band telephone set often uses alow-frequency component of 20 Hz to 50 Hz for communication, but the FIRfilter of the echo canceller cannot express low frequencies of thebroad-band telephone communication.

A simple solution to this problem is to increase the

tap length of the echo canceller to expand the frequency range that canbe expressed by the FIR filter (lower the frequencies that can beexpressed). An increased filter tap length, however, increases an amountof product-sum operations. As a result, when the echo canceller isimplemented by a digital signal processor, for example, increasedamounts of operations, increased hardware scale, and other adverseeffects cannot be avoided. If a filter that can express a frequency ofup to 20 Hz is desired at a sampling frequency of 16 kHz, animpractically huge filter with a tap length of 800 would be required. Ifthe input contains a frequency component that is too low to express, theecho canceller is affected by the low-frequency component that cannot beexpressed and behaves as if different offsets are applied in the timesegments “a” to “c”, as shown in FIG. 2B. Then, a corresponding offsetoccurs, and the average of the tap coefficient does not become zero asif the offset is superimposed (the offset exists).

The echo canceller primarily converges the tap coefficient to representthe transfer function of the echo path directly. In broad-band telephonecommunication, an offset of the tap coefficient makes it impossible forthe echo canceller to converge to the true essential transfer function.FIGS. 3A and 3B1 to 3B3 show the tap coefficient and the true transferfunction of the echo path when there is an offset. FIG. 3A shows themean values of the tap coefficients in the time segments “a” to “c”.FIGS. 3B1 to 3B3 show the converged tap coefficient of time segments “a”to “c”. In time segment “b”, the low frequency crosses the level ofzero, and no offset appears to exist in time segment “b”. As shown inFIGS. 3A and 3B1 to 3B3, a mismatch between the transfer function of thetrue echo path and the estimated tap coefficient degrades the echocancellation capability greatly.

Therefore, in the first embodiment, the offset removal section 13 andthe counter 16 are provided. The offset removal section 13 and thecounter 16 remove the offset of the tap coefficient, as described below.

The echo canceller 14 performs echo cancellation each time a new samplex(k) is input to the far-end input terminal 1, as indicated byexpression (3) above. The counter 16 counts that the creation of an echoreplica signal (accordingly, echo cancellation) is repeated for each ofthe samples input from the adaptive filter 15. The counter 16 outputs asignal indicating that the processing for the n samples has finished tothe offset removal section 13.

That is, this signal is output for every n samples.

The offset removal section 13 receives the n-sample timing signal fromthe counter 16 and performs offset removal in accordance with expression(7). The second term in the right-hand side of expression (7) representsthe offset component to be removed. Expression (7) indicates that theoffset is removed from the m-th tap coefficient at time k+1, i.e.,h(k+1,m) given by expression (2). The offset is obtained as the meanvalue of the M tap coefficients at time k+1 obtained by calculation.$\begin{matrix}{{h\left( {{k + 1},m} \right)} = {{h\left( {{k + 1},m} \right)} - {\frac{1}{M}{\sum\limits_{i = 0}^{M - 1}\quad{h\left( {{k + 1},i} \right)}}}}} & (7)\end{matrix}$

The offset removal section 13 may remove offset in accordance withexpression (8), instead of expression (7). The offset calculation ofexpression (8) differs from that of expression (7). In expression (7),the mean value of the M tap coefficients is calculated as a commonoffset for all the taps. In expression (8), an average on the time baseis calculated as an offset for each tap of the tap coefficient. Theoffset may also be calculated from the mean value on the time base andthe tap position. $\begin{matrix}{{h\left( {{k + 1},m} \right)} = {{h\left( {{k + 1},m} \right)} - {\frac{1}{n}{\sum\limits_{i = 0}^{n - 1}\quad{h\left( {{k + i},m} \right)}}}}} & (8)\end{matrix}$

Expression (7) or (8) for removing the offset component may not becarried out at the timing of h(k+1,m) and may be carried out at thetiming of h(k,m), of course. The value of n can be 160, for example, butthe value is not limited to 160. If 160 data samples of the far-endinput terminal 1 and the near-end input terminal 7 are processed as asingle frame, specifying the value of n to 160 is convenient. The valueof n may also be specified to the filter length (256, for example), ofcourse.

Actually, the offset removal as described above should be executed oncefor the number of samples, which is equal to the shortest tap length.The offset removal does not need to be executed for each sample. Then,the tap coefficient is updated in accordance with expression (2), andthe offset removal executed once will be reflected continuously. In thefirst embodiment, a bias signal (offset signal) is not directly removedfrom the input signal S_(in) to the near-end input terminal 7. This is adifference from the method described in the non-patent document 1.

FIGS. 4A, 4B1 to 4B3, and 4C1 to 4C3 (especially, FIGS. 4C1 to 4C3) showthe states shown in FIGS. 3A and 3B1 to 3B3 and the correction of alow-frequency offset in segments “a” to “c”. In segment “a”, thepositive offset of the tap coefficient is corrected to 0. In segment“c”, the negative offset of the tap coefficient is corrected to 0.

(A-3) Effects of First Embodiment

The offset of the tap coefficient is removed in each segment, asdescribed above, so that the echo component can be efficiently removedeven when a broad-band communication signal is used, thereby saving theamounts of operations and giving no effect other than echo cancellationto the transmission signal (near-end voice signal). The saved amounts ofoperations mean an increased processing speed if the echo canceller isimplemented by software utilizing a DSP or mean a reduced device scaleif the echo canceller is implemented by hardware.

(B) Second Embodiment

An echo canceller for removing a line echo according to the secondembodiment of the present invention will next be described in detailwith reference to drawings. FIG. 5 is a block diagram showing aconfiguration of an echo canceller 14A of the second embodiment and aperipheral configuration. Elements identical to those in the firstembodiment shown in FIG. 1 are denoted by the same reference marks.

As a comparison between FIG. 5 and FIG. 1 clearly shows, the echocanceller 14A of the second embodiment has a sending-side FFT section 20for performing a frequency analysis of the signal S_(in) input from thenear-end input terminal 7 and giving the result to the offset removalsection 22 and a receiving-side FFT section 21 for performing afrequency analysis of the signal R_(in) input from the far-end inputterminal 1 and giving the result to the offset removal section 22, inaddition to the elements of the first embodiment. Accordingly, thefunction of the offset removal section 22 differs a little from thefunction of the offset removal section 13 of the first embodiment.

A precondition for the first embodiment is that a call is made betweenbroad-band telephone sets.

However, a call between the near end and the far end can be made betweena broad-band telephone set and a telephone set having the conventionalband (conventional telephone set). The echo canceller 14A of the secondembodiment has the same effects as described in the first embodimenteven if the echo canceller is placed between unknown types of telephonesets at the near end and the far end, such as when a call is madebetween the conventional telephone set and a broad-band telephone set orbetween the conventional telephone sets. The offset of the low-frequencycomponent unique to the broad-band telephone signal is removedappropriately according to the telephone type.

First, the operation of the echo canceller 14A of the second embodimentwhen a broad-band telephone set is used at the far end and theconventional telephone set 5 is used at the near end will be described.

The signal R_(in) input from the far-end input terminal 1 is supplied tothe receiving-side FFT section 21. The receiving-side FFT section 21performs a frequency analysis of the input signal and detects thepresence or absence of either or both of a component of up to 300 Hz anda component of not less than 3400 Hz. The presence or absence of thecomponent can be detected by checking whether a power spectrum obtainedas a result of FFT exceeds a level of −30 dBm, for example. If acomponent of up to 300 Hz and/or a component of not less than 3400 Hz isfound (both or either of the conditions can be used), the receiving-sideFFT section 21 outputs an offset removal execution signal to the offsetremoval section 22, considering that a broad-band telephone set isconnected to the far end. When the offset removal execution signal isinput from either the receiving-side FFT section 21 or the sending-sideFFT section 20, the offset removal section 22 performs offset removal,as in the first embodiment. If the offset removal execution signal isgiven from the receiving-side FFT section 21, offset removal isexecuted. The operation does not depend on whether the sending-side FFTsection 20 gives an offset removal execution signal.

Next, the operation of the echo canceller 14A of the second embodimentwhen the conventional telephone set is used at the far end and thebroad-band telephone set is used at the near end will be described.

The signal S_(in) input from the near-end input terminal 7 is suppliedto the sending-side FFT section 20. The sending-side FFT section 20performs a frequency analysis of the input signal and detects thepresence or absence of both or either a component of up to 300 Hz and acomponent of not less than 3400 Hz. The presence or absence of thecomponent can be detected by checking whether a power spectrum obtainedas a result of FFT exceeds a level of −30 dBm, for example. If acomponent of up to 300 Hz or a component of not less than 3400 Hz isfound (both or either of the conditions can be used), the sending-sideFFT section 20 outputs an offset removal execution signal to the offsetremoval section 22 because a broad-band telephone set is connected atthe near end. The offset removal section 22 executes offset removal asin the first embodiment because the offset removal execution signal isinput from the sending-side FFT section 20. The operation does notdepend on whether the receiving-side FFT section 21 gives the offsetremoval execution signal.

The operation of the echo canceller 14A of the second embodiment whenthe conventional telephone sets are used at both the far end and thenear end will be described.

Both the sending-side FFT section 20 and the receiving-side FFT section21 output no offset removal execution signal to the offset removalsection 22 because the signal subjected to the frequency analysiscontains neither a component of up to 300 Hz nor a component of not lessthan 3400 Hz. The offset removal section 32 receives no offset removalexecution signal from the FFT sections 20 and 21 and does not executeoffset removal.

If the broad-band telephone sets are used at both the far end and thenear end, both the sending-side FFT section 20 and the receiving-sideFFT section 21 output the offset removal execution signal to the offsetremoval section 22, and offset removal as in the first embodiment is notexecuted.

The sending-side FFT section 20 can judge the receiving-side FFT section21 just once immediately after the channel is connected and may waitlater until the channel is re-connected. A continuous (both contiguousand intermittent) judgment may be made after the initial judgment.

According to the second embodiment, since the types of telephones usedat the far end and the near end are checked, and whether the offsetremoval of the tap coefficient is executed is determined by thecombination of the types of telephone sets used at the far end and thenear end, unnecessary operations can be avoided, the scale of operationof the digital signal processor (software processing entity) can bereduced, decreasing the power consumption, or the hardware scale can bereduced.

(C) Third Embodiment

An echo canceller for removing a line echo according to a thirdembodiment of the present invention will next be described in detailwith reference to drawings. FIG. 6 is a block diagram showing aconfiguration of an echo canceller 14B of the third embodiment and aperipheral configuration. The elements identical to the elements shownin FIG. 5 of the second embodiment described above are referenced by thesame reference marks.

As a comparison between FIG. 6 and FIG. 5 clearly shows, the thirdembodiment differs from the second embodiment in that the receiving-sideFFT section 21 is replaced by the receiving-side low-pass filter section31 and that the sending-side FFT section 20 is replaced by thesending-side low-pass filter section 30.

The sending-side low-pass filter section 30 and the receiving-sidelow-pass filter section 31 individually detect whether the suppliedsignal contains a component of up to 300 Hz (by detecting whether alevel obtained as a result of low-pass filtering exceeds a level of −30dBm). If a component of up to 300 Hz is detected, an offset removalexecution signal is output to the offset removal section 22.

The relationship between the combination of the types of telephones usedat the far end and the near end and whether the offset removal section22 executes offset removal of the tap coefficient is the same as in thesecond embodiment.

In the second embodiment described earlier, a frequency analysis by theFFT section is used to determine the type of the telephone set. It isknown that the FFT analyzes all the frequency range from thelow-frequency band-to the high-frequency band, depending on the numberof FFT window length samples. If a sampling frequency of 16000 Hz isused and 256 FFT window lengths are specified, positive frequencycomponents of 62.5 Hz to 8000 Hz (or 0 Hz to 7937.5 Hz) are equallycalculated. However, an offset important for the echo canceller is justin the very low frequency region. Therefore, detecting just thelow-frequency component and controlling offset removal in accordancewith the detected result is preferable in terms of saving the amounts ofoperations by the processor in digital signal processing and savingpower. Accordingly, the third embodiment uses a low-pass filter sectioninstead of the FFT section.

The third embodiment has the same effects as the second embodiment incontrolling offset removal in accordance with a combination of types oftelephone sets, and can also save greater amounts of unnecessaryoperations.

(D) Fourth Embodiment

An echo canceller for removing the line echo according to the fourthembodiment of the present invention will next be described in detailwith reference to drawings. FIG. 7 is a block diagram showing aconfiguration of an echo canceller 14C of the fourth embodiment andperipheral elements. The elements identical to the elements shown inFIG. 6 of the third embodiment described above are denoted by the samereference marks.

The fourth embodiment is the same as the third embodiment in that thesending-side low-pass filter section 40 and the receiving-side low-passfilter section 41 are provided. The fourth embodiment differs from thethird embodiment in that a low-pass filter in the sending-side low-passfilter section 40 and the receiving-side low-pass filter section 41 arevariable low-pass filters and the low-frequency component fordetermining whether to execute offset removal can be varied. An adaptivefilter 45 outputs information for determining the low-frequencycomponent of the sending-side low-pass filter section 40 and thereceiving-side low-pass filter section 41.

The reason why the sending-side low-pass filter section 40 and thereceiving-side low-pass filter section 41 use the variable configurationis as follows.

In the fourth embodiment, the amounts of operations of the low-passfilter are smaller than those in the third embodiment, so that theactual device can be set up easily, and the frequency of the low-passfilter is specified automatically.

In the third embodiment, a threshold level of 300 Hz is used to detect alow-frequency component, and whether to execute offset removal isdetermined accordingly. This determination method is useful for generaltelephone lines. If an independent private line is provided, the lowestallowable frequency of the line may not be 300 Hz, and the thirdembodiment may not be able to be applied. In this case, the cutofffrequency of the low-pass filter of the third embodiment must bechanged. On the other hand, the tap length of the filter section 12 ofthe echo canceller must be changed in accordance with the hybrid circuit4 (a type of the exchange device, and a distance to the hybrid circuit).

That is, the low frequency that can be represented by the echo cancellerdepends also on the tap length of the echo canceller. As has beendescribed above, the performance of the echo canceller will be degradedby the low-frequency component when there is a low-frequency componentthat cannot be expressed by the tap length of the filter sectionspecified in the echo canceller. The fourth embodiment is provided tocorrectly remove the offset as described below, irrespective of thevalue of low frequency allowed by the line or the filter tap length ofthe echo canceller.

In the fourth embodiment, whether a frequency cannot be expressed by thefilter section 12 of the echo canceller is detected as described laterby using a tap length as a parameter (see expression (8)). Accordingly,in the fourth embodiment, the echo cancellation capability is secured bydetecting whether any frequency cannot be represented automatically by aspecified tap length even if the tap length of the echo canceller ischanged and allowing optimum offset removal to be executed.

A person such as a designer specifies the tap length L of the filtersection 12 of the adaptive filter 45. As mentioned above, the designercan specify the tap length appropriately in consideration of the hybridcircuit 4. For example, a tap length of 256 can be used, but anotherappropriate setting can also be used. The adaptive filter 45 outputs thetap length L to the sending-side low-pass filter section 40 and thereceiving-side low-pass filter section 41.

In the sending-side low-pass filter section 40, the lower-limitfrequency LF (Hz) depends on the tap length L, as calculated fromexpression (9). In expression (9), sf denotes a sampling frequency,which can be 1600 Hz, for example.LF =sf/ (L −1)  (9)

The sending-side low-pass filter section 40 passes a frequency lowerthan the lower-limit frequency LF obtained from expression (9). Thereceiving-side low-pass filter section 41 also obtains the lower-limitfrequency LF from expression (9) and passes a frequency lower than thelower-limit frequency LF. The subsequent operation after the setting isthe same as in the third embodiment, and it is not described here.

The fourth embodiment has the same effects as the third embodiment.Because the sending-side low-pass filter section and the receiving-sidelow-pass filter section change the lower-limit frequency LF (Hz)automatically, keeping up with the variable setting of the tap length Lof the adaptive filter, so that the optimum offset removal can beautomatically performed even if the tap length of the echo canceller ischanged in accordance with the setup condition, and echo-free broad-bandcommunication can be implemented.

(E) Other Embodiments

A variable low-pass filter is used in the fourth embodiment. A variableconfiguration may be adopted when FFT and other frequency analysis isused, and the lower-limit frequency LF (Hz) determined by the number offilter taps may be specified as a detection threshold.

In the second to fourth embodiments, an FFT section or a low-pass filtersection is provided to process the frequency component of the receivedsignal and the sending signal. An FFT section or a low-pass filtersection for processing the frequency component of only the receivedsignal may be provided to control the removal of the offset component ofthe tap coefficient.

The above mentioned embodiments have been described with the intentionof applying them to broad-band VoIP communication. However, the presentinvention can be also applied to remove the echo caused by the offsetcomponent which is produced by an A/D converter as before in a call viaa public switched network, not using the VoIP.

Further, in the embodiments described above, the echo cancelleraccording to the present invention is used as an echo canceller forremoving the-line echo. The echo canceller can be also used to removeecho flowing from the microphone to the speaker.

Furthermore, in the embodiments described above, the offset of the tapcoefficient is obtained by averaging all the tap coefficients at thattime or by averaging the past tap coefficients. A median value, aweighed mean, or another calculation method may be applied. The number nof past tap coefficients which are used to calculate the offset of thetap coefficient may be varied with the set value L of taps of theadaptive filter, for example.

1. An echo canceller comprising: an echo replica forming means forforming an echo replica signal from a far-end input signal by using anadaptive filter including a filter section and a coefficient updatesection; an echo cancellation means for removing an echo component in anear-end input signal by subtracting the echo replica signal from thenear-end input signal; and an offset removal means for removing anoffset component produced under an effect of low frequencies from thefilter coefficient of the adaptive filter.
 2. The echo cancelleraccording to claim 1, wherein the offset removal means calculates a meanvalue of the filter coefficient of a tap length at a predeterminedtiming as an offset component and removes the offset component from thefilter coefficient of the adaptive filter.
 3. The echo cancelleraccording to claim 1, wherein the offset removal means calculates a meanvalue of the filter coefficients in a past predetermined period as anoffset component and removes the offset component from the filtercoefficient of the adaptive filter.
 4. The echo canceller according toclaim 2, wherein the offset removal means removes the offset componentonce in a predetermined period.
 5. The echo canceller according to claim1, further comprising a frequency component detection means fordetecting whether either or both of the far-end input signal and thenear-end input signal contain a low-frequency component lower than apredetermined frequency, wherein the offset removal means removes theoffset component when the frequency component detection means detectsthat a low-frequency component is contained.
 6. The echo cancelleraccording to claim 5, wherein the frequency component detection meansvaries the predetermined frequency in accordance with a set value of thetap length of the adaptive filter.
 7. The echo canceller according toclaim 3, wherein the offset removal means removes the offset componentonce in a predetermined period.